It's Sunday, and I'm only lacking about 3 characters. Though I understand the frustrations, I think it was a pretty good compromise. I wanted to unlock as many as possible before throwing a Smash party, and buying the game on Thursday night -- while still having a pretty busy few days -- I was able to get up to a 50+ roster by Saturday night, and it was pretty fun seeing new ones pop up throughout the party, with various people taking them on. Since the Switch is portable, it was easy to pop in between my errands and get an unlock, maybe a rematch. It's not perfect, but the system as it is does a decent job replicating the excitement of unlocking such a huge roster without the need of going through requirements that, previously, could get pretty ridiculous.
One thing's for sure, though: I'm glad to hear that other people think the challengers are surprisingly hard to beat, because I was starting to think I just really sucked at Smash Bros. all of the sudden.